1. Field of the Invention
The present invention relates to an optical distribution network system for operating DBA (Dynamic Bandwidth Assignment) in a duplex optical distribution section such as a PDS (Passive Double Star) section.
2. Description of Related Art
A conventional optical distribution network system is disclosed in Japanese patent application laid-open No. 11-122172/1999, or specified in ITU-T (International Telecommunication Union-Telecommunication) Recommendation G.983.1, for example.
FIG. 11 is a diagram showing a conventional optical distribution network system defined in ITU-T Recommendation G.983.1. In this figure, the reference numeral 1 designates an optical line termination (abbreviated to “OLT” from now on), reference numerals 2-1–2-n each designate an optical network unit (abbreviated to “ONU” from now on), and 3 designates an optical splitter.
Next, the operation of the conventional system will be described.
In the ITU-T Recommendation G.983.1, a downstream optical signal from the OLT 1 is split by the optical splitter 3 to be broadcast to the ONUs 2-1–2-n. 
On the other hand, upstream signals from the ONUs 2-1–2-n are multiplexed by the optical splitter 3 to be transmitted to the OLT 1. In the course of this, to multiplex the upstream signals from the ONUs 2-1–2-n on the optical splitter 3, access control (delay control) is carried out. The delay control is also described in the ITU-T Recommendation G.983.1.
FIG. 12 is a block diagram showing a detailed configuration of the optical distribution network system of FIG. 11. In this figure, the reference numeral 11 designates a delay measurement cell generation controller, 12 designates an OAM (Operation Administration and Maintenance) cell multiplexer, 13 designates a transmitting/receiving section, 14 designates a state controller, 15 designates an OAM cell demultiplexer, 16 designates a delay measurement section, 17 designates a delay correcting section, 21 designates a transmitting/receiving section, 22 designates a frame synchronization section, 23 designates an OAM cell demultiplexer, 24 designates a delay setting section, 25 designates a buffer memory, 26 designates a state controller, and 27 designates an OAM cell multiplexer.
The optical distribution network system as shown in FIG. 12 carries out a sequence called ranging at each start-up of the ONU.
The ranging is carried out as follows. First, in the OLT 1, the delay measurement cell generation controller 11 generates delay measurement cells for particular ONUs 2-1–2-n. 
The grant of the delay measurement cells generated by the delay measurement cell generation controller 11 are each multiplexed into downstream main data as an OAM cell by the OAM cell multiplexer 12 to be transmitted to the ONUs 2-1–2-n through the transmitting/receiving section 13 including an optical transceiver and a WDM (Wavelength Division Multiplexing) coupler.
Each of the ONUs 2-1–2-n converts the received optical signal to an electric signal by the transmitting/receiving section 21 including an optical transceiver and a WDM coupler.
The electric signal is fed to the frame synchronization section 22 that regularly inserts frame synchronization bits into OAM cells, which enable the frame synchronization to be established and the cell delimiter of each cell to be identified.
For example, the OAM cell demultiplexer 23 of the ONU 2-1 identifies incoming data cells and OAM cells, and separates them. The delay setting section 24, recognizing the grant of delay measurement cells in the isolated OAM cells, immediately notifies the OAM cell multiplexer 27 of it to transmit a delay measurement cell as a response to the OLT 1 via the transmitting/receiving section 21 and the optical splitter 3. Thus, receiving the delay measurement cell, the ONU 2-1 sends the response immediately back to the OLT 1.
On the other hand, the OAM cell demultiplexer 15 of the OLT 1 separates the OAM cells from the data cell.
The delay measurement section 16, detecting the delay measurement cell separated by the OAM cell demultiplexer 15, measures a round-trip delay by the response of the delay measurement cell. The round-trip delay is a time period between the transmission and reception of the cell by the OLT 1, during which the cell is transmitted to the ONU 2-1 via the optical splitter 3, and is sent back to the OLT 1.
The delay measurement cell generation controller 11 computes the delay between the OLT 1 and the ONU 2-1 from the round-trip delay, generates a delay measurement value information including information about the delay, and supplies it to the OAM cell multiplexer 12. The OAM cell multiplexer 12 inserts the delay measurement value information to an OAM cell to be transmitted to the ONU 2-1 by the transmitting/receiving section 13.
Receiving the OAM cell including the delay measurement value information, the OAM cell demultiplexer 23 of the ONU 2-1 isolates the OAM cell. When the OAM cell includes the delay measurement value information, the delay setting section 24 controls the beginning of the cell reading from the buffer memory 25 with this delay measurement value information. Thus, the multiple ONUs can each set the transmission timing to the OLT 1 considering the delay time, so that the multiplexing can be performed in order, and the upstream optical transmission is carried out normally. The state controllers 14 and 26 make a decision that the party is in an operating state when the cell is sent back within the normal location, followed by measuring the delay amount of the upstream cell, by fine adjustment of the delay amount of the cell by the delay correcting section 17, whereas when the cell is not sent back within the normal location, they make a decision that the party is in an abnormal condition.
The ITU-T Recommendation G.983.1 also defines a duplex optical distribution network system as shown in FIG. 13, which completely doubles the OLTs, the ONUs and the components between them. The duplex optical distribution network system comprises instead of the OLT 1 as shown in FIG. 11, an OLT 1a as an working side and an OLT 1b as a standby side, which are connected to the optical splitters 3a and 3b. In addition, instead of the ONUs 2-1–2-n as shown in FIG. 11, it comprises ONUs 2-1a–2-na as the working side, and ONU 2-1b–2-nb as the standby side, which are connected to the optical splitters 3a and 3b. 
Then, optical fibers interconnect the optical splitters 3a and 3b with the OLTs 1a and 1b, and the optical splitters 3a and 3b with the ONUs 2-1a–2-na and 2-1b–2-nb. 
The duplex optical distribution network system sometimes uses a technique called DBA (Dynamic Bandwidth Assignment) that operates as follows:
FIG. 14 is a diagram illustrating an outline of the bandwidth assignment by the DBA.
As for each of the ONUs 2-1a–2-na, a minimum cell rate (an available traffic bandwidth without exception) and a peak cell rate (a traffic bandwidth of a maximum possible transmission which is not necessarily assured) are set by contract.
When a 0-system is the working side, the sum total of the minimum cell rates of the ONUs 2-1a–2-na are secured on the 0-system transmission line without fail. A usable bandwidth for DBA (the total transmission capacity—the sum total of the minimum cell rates of the ONUs) as shown in FIG. 14 can be used in common by the ONUs 2-1a–2-na. 
If an upstream cell bandwidth from the ONU is about to exceed the established bandwidth (equals the minimum cell rate here), a bandwidth monitor in the OLT 1a installed for each of the ONUs 2-1a–2-na detects it. For example, the bandwidth monitor measures the bandwidth of the cells by counting the number of incoming cells in a fixed time period, which are transmitted from each of the ONUs 2-1a–2-na to the OLT.
Then, the OLT 1a increases the bandwidth of the ONU within the usable bandwidth for DBA in such a way that the OLT 1a notifies the ONUs 2-1a–2-na of the reassigned bandwidth so that the ONUs 2-1a–2-na can change the transmission traffic bandwidth. This enables the OLT 1a to dynamically assign additional bandwidths to some ONUs 2-1a–2-na that require a bandwidth greater than the minimum cell rate.
When the ONUs 2-1a–2-na that are assigned the additional bandwidths are congested, the congested ONUs apportion the bandwidths among them within the usable bandwidth for DBA.
Thus, in the event of the congestion, not all the congested ONUs can secure a sufficient bandwidth because the sum total of the peak cell rates of the ONUs 2-1a–2-na would exceed the total transmission capacity in such a case.
In contrast, when a particular ONU decreases its upstream cell bandwidth below the increased bandwidth, it can be reduced with ensuring the minimum cell rate.
With the foregoing configuration where the OLTs and the ONUs are duplexed as shown in FIG. 13, the conventional optical distribution network system secures the transmission bandwidths of all the ONUs 2-1–2-n within the bandwidth of the 0-system OLT 1a (when the 0-system is the working side) at the startup of the system. Accordingly, the maximum additional bandwidth available by the DBA equals the total transmission bandwidth of the working side minus the sum total of the minimum cell rates of the ONUs 2-1a–2-na. This presents a problem of being unable to secure a large usable bandwidth for DBA when the ONUs 2-1a–2-na congest because of increasing bandwidths (because the bandwidth of the 1-system remains unused).